It is known that amorphous films composed of alloys consisting of transition metals, such as iron and cobalt, and rare earth elements, such as terbium (Tb) and gadolinium (Gd), have a magnetic easy axis perpendicular to the film and are capable of forming a small inverse magnetic domain with magnetization anti-parallel to the magnetization of the film. By corresponding the existence or nonexistence of this inverse magnetic domain to "1" or "0", it becomes possible to record a digital signal on such amorphous films as mentioned above.
As amorphous films composed of transition metals and rare earth elements which are used as magnetooptical recording media, there is disclosed an amorphous film of Tb--Fe alloy system containing 15-30 atom % of Tb, for example, in Japanese Patent Publication No. 20691/1982. Tb, used in this amorphous film, is classified among so-called heavy rare earth elements in the rare earth elements. Magnetooptical recording media comprising alloys composed of such heavy rare earth elements as mentioned above and transition metals such as iron and cobalt have been widely investigated.
However, no extensive researches have been made on magnetooptical recording media comprising alloys composed of so-called light rare earth elements such as europium (Eu) and the like which equally belong to the class of rare earth element, and transition metals. For instance, J. Appl. Phys. 57 3906 (1985) barely contains a technical report on such magnetooptical recording media of the type as referred to above, said report disclosing that amorphous films of Nd--Fe alloy system and Nd--Fe--Co alloy system are usable as magnetooptical recording media. Great hopes are entertained of magnetooptical recording media comprising amorphous films of Nd--Fe or Nd--Fe--Co alloy system since Nd is relatively inexpensive and, at the same time, the recording media have large kerr-rotation angle .theta.K and have excellent readout performance (C/N ratio).
In the amorphous films of Nd--Fe or Nd--Fe--Co alloy system referred to above, however, there was involved such a problem that no films with a magnetization with easy axis perpendicular to the film can be formed unless a substrate on which the films are to be formed is heated at the time of forming said films. (See, for example, MAG-51-91p17 (1985), a research material by a society for the study of magnetics, the Electric Society of Japan.) Accordingly, if no films with a magnetization with easy axis perpendicular to the film can be formed on a substrate unless said substrate is heated, it follows that no films with a magnetization with easy axis perpendicular to the film can be formed on a substrate composed of an organic materials poor in heat resistance, such as plastics.
These amorphous films of Nd--Fe or Nd--Fe--Co alloy system were further desired to be improved in magnetic anisotropic energy in direction normal to the film Ku, coercive force Hc and saturation magnetization Ms, though said films have excellent magnetic characteristics.
In the amorphous films of Nd--Fe or Nd--Fe--Co alloy system, moreover, there was involved such problem that there is a fear of lowering magnetic properties of the films owing to oxidation of said films because they contain Fe as one of constituent components thereof.
Japanese Patent L-O-P Publication No. 165847/1986 discloses magnetooptical recording media comprising films of alloy composed of at least one light rare earth metal selected from among Sm, Nd, Pr and Ce, at least one transition metal selected from among Fe, Ni and Co, and at least one heavy rare earth metal selected from among Tb, Gd and Dy, said films being represented by the composition formula [(Sm, Nd, Pr, Ce).sub.1-x (Tb, Gd, Dy).sub.x ] .sub.y (Fe, Co, Ni).sub.1-y wherein 0&lt;x.ltoreq.0.5 and 0.1.ltoreq.y.ltoreq.0.4. This publication discloses as an example a magnetooptical recording media comprising a film represented by the composition formula (Nd.sub.x Dy.sub.1-x).sub.0.25 Fe.sub.0.75.
In Japanese Patent L-O-P Publication No. 165847/1986 referred to above, however, there is disclosed concretely only Nd--Dy--Fe type magnetooptical recording medium. In this connection, the present inventors have found that the recording media disclosed in the said publication involve such a serious problem that there is a fear of disappearance of the information recorded on said recording media when they are particularly of Nd--Dy--Co type having the composition falling within the abovementioned range since the resulting films are small in coercive force, though they have satisfactory values of kerr-rotation angle, and consequently are adversely affected by external magnetic field.
As a result of extensive researches conducted with a view of solving the above-mentioned problems at a stroke, the present inventors have finally found that these problems can be solved in magnetooptical recording media comprising amorphous films of Nd--Fe or Nd--Fe--Co alloy system having been further incorporated with specific amounts of specific heavy rare earth elements, or in magnetooptical recording media comprising novel amorphous films of Nd--Dy--Co alloy system. Thus, the present invention has been accomplished on the bases of the above finding.
The present invention is intended to solve such problems associated with the prior art as mentioned above, and an object thereof is to provide magnetooptical recording media comprising amorphous films of alloy and processes for producing the same, which amorphous films of alloy have no fear of lowering magnetic characteristics owing to oxidation of said films, are capable of forming a film with a magnetization with easy axis perpendicular to the film having excellent magnetic properties on a substrate even without heating the substrate, and are excellent in coercive force.